I designed a circuit similar to this
one a long time ago to help a beekeeper count the number of bees going into or out
of a hive. The low power circuit uses a slotted opto-sensor to detect the passing
bees. The circuit advances an electronic counting module whenever a honeybee passes
through the sensor. The device only counts the number of bees going through the
sensor. A different circuit would be needed to count the number of bees only going
out or only coming into the hive.